Bonding Method for Thin Film Diamond Providing Low Vapor Pressure at High Temperature

ABSTRACT

A thin diamond film bonded to a diamond substrate made by the process of heating a diamond substrate inside a vacuum chamber to about 500° C., cooling the diamond substrate, coating a first surface of the diamond substrate with chromium, depositing an initial layer of palladium, heating the diamond substrate, allowing the chromium and the diamond substrate to form a chemical bond, inter-diffusing the adhesion layer of chromium and the initial layer of palladium, cooling, depositing palladium, placing a shadow mask, degassing the vacuum, depositing a tin layer, assembling the tin layer, heating the tin layer, melting the tin layer, and bonding the thin diamond film to the diamond substrate. A thin diamond film bonded to a diamond substrate comprising a thin diamond film, a layer of chromium, palladium, tin, and a diamond substrate.

This application claims priority to and the benefits of U.S. PatentApplication No. 61/864,854 filed on Aug. 12, 2013, and U.S. patentapplication Ser. No. 14/306,062 filed on Jun. 16, 2014, the entiretiesof both are herein incorporated by reference.

BACKGROUND

This disclosure concerns a method of bonding a thin film of diamond to asecond thick diamond substrate in a way that does not cause the exposed(un-bonded) diamond surface to become contaminated by the bondingprocess or when the bonded diamond is held at high temperature for manyhours in vacuum.

In addition, this process allows the thin film diamond to be bonded overa hole in the substrate diamond.

In addition, the process provides a metal surface suitable for wirebonding on the surface of the thick diamond substrate adjacent to thethin film diamond but electrically isolated from it.

In addition, the process provides a bond material that is inert enoughto withstand subsequent processing such as photo-resist development andmost chemical etches.

Common bonding methods include thermo-compression bonding and brazing.Thermo-compression bonding requires application of very high pressure athigh temperature, and is thus difficult to accomplish for a fragilepart. More fundamentally, metals such as Au, that are most commonly usedfor thermo-compression bonding, exhibit fast surface diffusion, whichcould create an electric short circuit and/or contaminate the NEAsurface.

Brazing requires that a filler metal be melted to create the bond, suchthat the melting point of the metal must be well above the intendedoperating temperature. However, the vapor pressures of suitable metalsat their melting points are high enough to contaminate the diamond. Whencontaminated with metals at high temperature, the diamond typicallyreacts with the metal, forming metal carbides that are relatively inertand difficult to remove chemically without also removing the bond metal.For example, braze tests using Cu:Ag (72:28 wt %) eutectic alloy, whichmelts at 779° C., resulted in large amounts of both Cu and Ag on theexposed diamond surface, and these metals could not be completelyremoved using solutions that dissolve pure Cu and pure Ag. Other metalsand alloys also have high vapor pressure at their melting points. Thusbrazing is not an appropriate bonding method.

BRIEF SUMMARY OF THE INVENTION

The purpose of the invention is to bond a thin film of diamond to asecond thick diamond substrate in a way that does not cause the exposed(un-bonded) diamond surface to become contaminated by the bondingprocess or when the bonded diamond is held at high temperature for manyhours in vacuum.

In addition, the process allows the thin film diamond to be bonded overa hole in the substrate diamond.

In addition, the process provides a metal surface suitable for wirebonding on the surface of the thick diamond substrate adjacent to thethin film diamond but electrically isolated from it.

In addition, the process provides a bond material that is inert enoughto withstand subsequent processing such as photo-resist development andmost chemical etches.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates a possible arrangement for the bonding method of thinfilm diamond.

FIG. 2 illustrates vapor pressures of relevant metals.

FIG. 3 illustrates weight percent of tin as a function of temperature.

DETAILED DESCRIPTION

This disclosure describes a method to bond a thin film of diamond to asecond thick diamond substrate in a way that does not cause the exposed(un-bonded) diamond surface to become contaminated by the bondingprocess. Furthermore, this method avoids causing the exposed (un-bonded)diamond surface to become contaminated when the bonded diamond is heldat high temperature for many hours in vacuum.

In addition, the process allows the thin film diamond to be bonded overa hole in the substrate diamond. In addition, the process provides ametal surface suitable for wire bonding on the surface of the thickdiamond substrate adjacent to the thin film diamond but electricallyisolated from it. In addition, the process provides a bond material thatis inert enough to withstand subsequent processing such as photo-resistdevelopment and most chemical etches.

Diamond surfaces terminated with hydrogen atoms exhibit the property ofnegative electron affinity (NEA) and are also chemically stable,potentially making diamond a key material in electron sources.

A fraction of a single atomic layer of other atoms or moleculesreplacing or coating the hydrogen atoms on the diamond surface candegrade or destroy the NEA property; hence procedures to establish andmaintain the hydrogen terminated surface free of contamination duringuse must be found in order to utilize NEA diamond in practice.

A particular type of electron source which has been called a “Thindiamond electron beam amplifier” (U.S. Pat. No. 6,060,839) requires thinfilms of diamond typically less than 25 microns (0.001″) thick. Theelectron beam amplifier requires that one side of the thin diamond filmbe coated with a thin conducting layer and exposed to a primary electronbeam, while the other side, having NEA, is held at a positive potentialwith respect to the coated side. In practice the thin diamond film mustbe bonded over a hole in a physically robust and thermally conductivesubstrate to facilitate handling, provide electric contact, allowexposure to the primary electron beam, and to remove heat generated bythe device in operation. To reduce stress caused by thermal expansion aswell as maximize thermal transport, it is best to fabricate thesubstrate from diamond.

During use as an electron source, the diamond film is heated by theprimary electron beam and by internal currents, such that the maximumemission current may be limited by the maximum temperature that can besustained without damage. The elevated temperature also ensures thatmolecules such as water or carbon monoxide will not be adsorbed duringuse.

Implementing the diamond electron beam amplifier thus creates severalrequirements for the bond and bonding process: The bonding material mustbe an electric conductor, and, if the diamond substrate is insulating,the bond material must extend from underneath the thin diamond to apoint where it can be contacted. The bonding material should not coatthe top surface or edges of the thin diamond film, which would create anelectric short circuit. The bond must sustain temperatures of at least400° C. without melting, decomposing or significant sublimation. Thesublimation rate should be low enough to prevent the diamond surfacefrom becoming contaminated as the device is operated at high temperaturefor many hours.

To fabricate bonds for use with a thin diamond electron beam amplifier,we utilized a form of “Transient Liquid Phase” (TLP) bonding. In TLPbonding a low melting point metal and a high melting point metal areeach deposited onto the surfaces to be bonded. Then the surfaces arebrought into contact while the joint is heated above the lower meltingpoint to create liquid which will flow and fill any gaps. At theelevated temperature the two metals inter-diffuse and/or react to forman alloy or compound with a higher melting point, such that the bondmaterial will no longer melt when heated well above the bondingtemperature. In this way a bond that can withstand high temperature iscreated at low temperature.

We have implemented palladium (Pd) and tin (Sn) as the high and lowtemperature melting point materials. Sn is chosen because it has a lowmelting point as well as low vapor pressure. Pd is chosen because it haslow vapor pressure, has a relatively high solubility for Sn, and becauseit is relatively inert, making it chemically compatible with laterprocessing.

Thus after the bond is finished, the Sn is dissolved in the Pd withoutforming brittle or chemically reactive compounds. Sn melts at 230° C.and has a vapor pressure of 10⁻¹² ton at 500° C. Pd melts at 1555° C.and has a vapor pressure of 10⁻¹² ton at 610° C.

Sn is soluble in Pd up to 18 wt %, such that no Sn-Pd intermetalliccompounds are formed when the weight fraction of Sn is less than 18%.The vapor pressure of Sn dissolved in Pd is not known; Raoult's law forsolutions suggests the dissolved Sn vapor pressure would be reduced tono more than 18% of the value for pure Sn, however because a number ofPdSn compounds form, the vapor pressure of the dissolved Sn is likely tobe significantly lower than Raoult's law predicts.

Palladium containing up to 18 wt % tin is chemically and physicallysimilar to pure Pd, although its melting point is reduced to 1280° C.The alloy is not attacked by many standard chemicals including organics,ammonium hydroxide +hydrogen peroxide, hydrochloric acid +hydrogenperoxide, hydrofluoric acid, or sulfuric acid. Pure Sn and most commoncontaminants can be removed using the above solutions. Both pure Pd andthe alloy can be etched in hot nitric acid or nitro-hydrochloric acid(aqua-regia).

Analysis of the bow/warp and roughness of the polycrystalline diamondsubstrate material showed the gap between the film and substrate may beas much as 200 nm, so the Sn layer must be much more than 200 nm thick.In practice we use 500 nm thick Sn layers. To sufficiently dilute the Snin Pd, the Pd must be at least 2000 nm thick. In practice we use 2500 nmthick Pd layers.

EXAMPLE

Since neither Pd nor Sn will react readily with diamond, we first coatthe diamond surfaces in the areas to be bonded with an adhesion layer ofChromium (Cr). The Cr layer also forms an electric contact to the thindiamond film, needed to remove charge generated by the primary electronbeam.

The Cr is thin enough to allow the primary electron beam to pass throughit without excessive energy loss. The Cr layer thickness can be variedsomewhat depending on the intended primary beam energy and current. Athinner layer may be used for very low primary beam energies or athicker layer can be used to reduce resistive loss.

The Cr coating on the substrate is patterned using a shadow mask; the Crcoating on the thin diamond film is not patterned. Before depositing theCr using a magnetron sputter source, we remove adsorbed gases from thediamond by heating inside the vacuum chamber to 500° C. until thepressure drops below 2×10⁻⁷ ton. This step ensures the Cr willchemically react with the diamond rather than the adsorbed gasmolecules. Then we reduce the diamond temperature to ˜200° C. anddeposit 20 nm Cr. Deposition should not be carried out at highertemperatures because the Cr may move under the shadow mask before it isadsorbed.

Pd is deposited onto the substrate diamond after the Cr adhesion layer,with the same shadow mask, without breaking vacuum. An initial layer of300 nm of Pd is deposited on top of the Cr. Then the diamond temperatureis brought up briefly to 600° C. to cause the Cr and C to form aphysically strong chemical bond and the Cr and Pd to inter-diffuse. Thediamond temperature is then reduced back to 200° C. and 2200 nm of Pd isdeposited, for a total Pd thickness of 2500 nm.

A very small amount of Pd may get under the shadow mask, and can beremoved with a brief nitric acid etch.

Before depositing Sn onto the thin diamond film over the Cr coating, ashadow mask is placed over the film covering the center portion of thefilm, where the primary electron beam will strike. The shadow mask alsokeeps the Sn layer away from the film edges, reducing the exposed areaand thus reducing the sublimation rate of the bond. Sn is deposited atroom temperature, following a vacuum degas at 200° C. to minimizesublimation of the deposited Cr. The Sn layer is deposited to athickness of 500 nm. The Sn layer must be deposited only shortly (<8hrs) before bonding, or stored under vacuum or inert gas, such that airexposure and oxidation are minimized.

To create the bond, the two coated pieces are assembled and aligned,then rapidly heated in inert atmosphere. Rapid heating is necessary tocause the Sn to melt before it inter-diffuses with the Pd. Slow heatingcauses inter-diffusion to occur at the contact points, forming a highmelting point alloy without significant adhesion.

To assemble and align the diamond substrate and film, we use a jig,shown in FIG. 1. The substrate is placed on a stainless steel diskprovided with posts. Notches cut in the edge of the substrate align withtwo posts in the disk. A sheet of diamond with similar edge notches anda central hole matching the shape of the diamond flake is placed overthe substrate and aligned using the posts. The Sn-coated side of thediamond film is placed over the Pd-coated side of the substrate film andaligned using the diamond sheet. A smaller diamond sheet with a centralhole is placed over the film. The assembly is placed on top of a heatermade from a molybdenum body in the shape of a hollow cylinder and filledwith a tungsten coil heater and alumina potting (Heatwave Labs model101133). The heater is mounted inside a stainless steel vacuum chamber.A ceramic ball is placed over the hole in the smaller diamond sheet, anda cup-shaped molybdenum weight, bearing on the ball, is placed over theassembly. The sides of the weight extend over the outside of the heaterto provide alignment and promote heat transfer. Type K thermocouples areattached to the heater and the weight. The thermocouple attached to theheater is used to monitor the process as described below. Thethermocouple attached to the cap shows a substantially lower temperature(approximately half the heater temperature) during the initial heatingand approximately 50° C. cooler in steady state. To power the heater, weuse a temperature controller (Stanford Research Systems model PTC10) toregulate primary power to a 12.5 V, 20A transformer (-175W).

The chamber is evacuated below 2×10⁻⁷ torr, the heater is held at 100°C. until the pressure is again below 2×10⁻⁷ ton, then the heater iscooled in vacuum. Prior to the final heating step, the chamber isback-filled with purified Ar gas to 7 torr. Initially we apply fullpower continuously to the heater, causing the temperature to increase asrapidly as possible. We achieve about 10° C./s or 600° C. in 60 seconds.The heater power is turned off as the heater temperature passes 600° C.The temperature continues to rise to about 615° C., then begins to cool.The heater is allowed to cool to 450° C. and held at 450° C. for 1 hour,then allowed to cool to room temperature.

We use X-ray photoemission (Thermo Scientific model K-Alfa) to check theexposed diamond surface for contamination. Using the heating protocolabove we find no contamination on the diamond film after bonding. Thelarge temperature difference between the melting point of Sn (230° C.)and the heater temperature is needed to provide rapid heat transferbetween heater and sample. The Ar gas backfill is used to promote fasterheat transfer. The backfill pressure is limited to several torr to avoidexcessive heat transfer to the chamber walls, which would release gasfrom the chamber walls. If the peak temperature reaches 700° C. thediamond film may be contaminated with small amounts of evaporated Sn. Apeak temperature of 500° C. will result in no bond or an incompletebond. The 1 hour hold time is thought to be much longer than required toensure complete and homogeneous inter-diffusion of Pd and Sn. Using theprotocol above, it is possible to produce a bonded flake free of metalcontamination. If the peak temperature is exceeded, any Sn contaminationcan be removed by submersion in concentrated sulfuric acid at 200° C.for 60 seconds.

This new bonding process involves the bond materials having very lowvapor pressure and the bonding process leaves the exposed surfaces freeof contamination to less than 1% of a monolayer. The claimed processmight also be used to bond thin diamond for other purposes, for examplevacuum windows for radiation (for example, in the microwave throughvisible frequencies) are also likely to be sensitive to surfacecontamination. Another application for thin diamond is vacuum windowsfor x-ray radiation. Coated with a layer of Al or other metal, thediamond films could be used as x-ray anodes that double as x-raywindows.

A very similar process would substitute pure Copper (Cu) or Pd alloyswith Cu, Ni, or Pt for pure Pd. Cu is less chemically inert and hashigher vapor pressure than Pd; however it is less expensive and willform excellent bonds. Ni:Pd, Pt:Pd, or Ni:Pd:Pt alloys may provide evenlower vapor pressure and may reduce the required heating rate.Substituting pure Ni for Pd will form a bond with low vapor pressure,but the Ni and Sn react to form compounds that are relatively brittleand less inert than Pd. An alternative may be to slow the Sn oxidation(and thus allow longer exposure to air) by coating it with a thin layerof Pt.

Many modifications and variations of the present invention are possiblein light of the above teachings. It is therefore to be understood thatthe claimed invention may be practiced otherwise than as specificallydescribed. Any reference to claim elements in the singular, e.g., usingthe articles “a,” “an,” “the,” or “said” is not construed as limitingthe element to the singular.

What we claim is:
 1. A thin diamond film bonded to a diamond substratemade by the process of: heating a diamond substrate inside a vacuumchamber to about 500° C.; cooling the diamond substrate to a temperatureof about 200° C.; coating a first surface of the diamond substrate withan adhesion layer of chromium; depositing an initial layer of palladiumon the adhesion layer of chromium; heating the diamond substrate to atemperature of about 600° C.; allowing the chromium and the diamondsubstrate to form a chemical bond; inter-diffusing the adhesion layer ofchromium and the initial layer of palladium; cooling the diamondsubstrate to about 200° C.; depositing a second layer of palladium;placing a shadow mask on a thin diamond film; degassing the vacuum atabout 200° C. and minimizing sublimation of the deposited chromium;depositing a tin layer onto the thin diamond film; assembling the tinlayer on the thin diamond film with the second palladium layer; heatingthe tin layer on the thin diamond film and the second palladium layer;melting the tin layer; and bonding the thin diamond film to the diamondsubstrate.
 2. The thin diamond film bonded to a diamond substrate as inclaim 1 wherein the step of coating a first surface of the diamondsubstrate with an adhesion layer of chromium results in an adhesionlayer of chromium that has a thickness of about 20 nm and wherein thetin layer deposited has a thickness of about 500 nm.
 3. The thin diamondfilm bonded to a diamond substrate as in claim 1 wherein the step ofdepositing an initial layer of palladium further includes the step ofdepositing an initial layer of about 300 nm of palladium on the adhesionlayer of chromium and wherein the step of depositing a second layer ofpalladium further includes the step of depositing about 2200 nm ofpalladium and thereby resulting in a total palladium thickness of about2500 nm.
 4. The thin diamond film bonded to a diamond substrate as inclaim 3 further including the step of forming an electric contactbetween the chromium layer and the thin diamond film.
 5. A thin diamondfilm bonded to a diamond substrate comprising: a thin diamond film; alayer of chromium; a layer of palladium; a layer of tin; and a diamondsubstrate.
 6. The thin diamond film bonded to a diamond substrate ofclaim 5 further including a layer of gold on the layer of tin.
 7. Thethin diamond film bonded to a diamond substrate of claim 5 wherein thelayer of chromium forms an electric contact to the thin diamond film andremoves charge generated by a primary electron beam.
 8. The thin diamondfilm bonded to a diamond substrate of claim 5 wherein the layer of tinis soluble in the layer of palladium to about 18 wt % and wherein thindiamond film and the diamond substrate are bonded physically,electrically, and thermally without contamination.
 9. The thin diamondfilm bonded to a diamond substrate of claim 5 further comprising a layerof Al or other metal and wherein the thin diamond film is a x-ray anodeand a x-ray window.
 10. The thin diamond film bonded to a diamondsubstrate of claim 7 wherein the layer of chromium has a thickness ofabout 20 nm.
 11. The thin diamond film bonded to a diamond substrate ofclaim 7 wherein the layer of palladium has a thickness of between about300 nm and about 2500 nm on the layer of chromium.
 12. The thin diamondfilm bonded to a diamond substrate of claim 11 wherein the tin layer hasa thickness of about 500 nm.